This application relates to polycarbonate blends which have enhanced thermal properties as compared to BPA and to a method of making same.
Polycarbonates are a well known class of high impact resistant thermoplastic resins characterized by optical clarity, high ductility as well as other advantageous properties. They are frequently employed as lenses and windows as a result of their transparency.
Bisphenol A (BPA) polycarbonate is the predominant commercially available resin of this type. It is derived from 2,2-bis(4-hydroxyphenyl)propane and ordinarily has a glass transition temperature of about 150xc2x0 C. It is of increasing interest to prepare polycarbonates which, while retaining the ductility of BPA, have higher glass transition temperatures and are therefore more resistant to softening when heated. Polycarbonates possessing increased glass transition temperatures are very desirable, for instance, in the automotive and aircraft industries. Particularly, they may be used in the preparation of automotive headlamp lenses which are becoming smaller in size and characterized by closer proximity of the lenses to the heat-generating light source.
Current approaches to high heat polycarbonate involve polymerizing BPA with a monomer with that increases the copolymer glass transition temperature (Tg). This improvement usually comes at the expense of other polymer properties. For example, Bayer has commercialized a family of copolymers based on Bisphenol I, which provide higher Tg at the expense of reduced ductility. Bayer APEC DP9-9340 resin gives a Tg of 177xc2x0 C. and a 23xc2x0 C. notched izod impact of 6 ft-lb/in, as compared to a Tg of 150xc2x0 C. and a 23xc2x0 C. notched izod impact of 14-16 ft-lbs/in for BPA homopolymer.
U.S. Pat. No. 5,480,959 disclosed substantially pure bisphenols; in particular, 4-[1-[3-(4-hydroxyphenyl)-4-methylcyclohexyl]-1 -methylethyl]phenol (hereinafter referred to as BPT-1) and 4,4xe2x80x2-[1-methyl-4-(1-methylethyl)-1,3-cyclohexandiyl]bisphenol (hereinafter referred to as BPT-2). These materials can be used to make homopolycarbonates that possess increased glass transition temperatures on the order of about 198xc2x0 C. when employing BPT-1 and about 249xc2x0 C. when employing BPT-2. Unfortunately, these materials do not have the toughness of BPA polycarbonates.
A copolymer of BPT-1 and BPA (35:65 mol %) was reported to have a glass transition temperature of 171xc2x0 C. and a 23xc2x0 C. notched Izod impact strength of about 3.8 ft-lbs/in. The production of copolymers to attain specific characteristics is not optimum, however, because it requires a separate manufacturing run for each type of product. This increases the cost. Thus, a preferred approach to the manufacture of products with a spectrum of characteristic properties is to have a limited number of feedstock polymers which can be blended to arrive at mixtures with the desired properties. In the case of BPA polycarbonates, one seeming option would be mixing BPA polycarbonates with BPT-1 or BPT-2 polycarbonates to arrive at blends having high Tg values. This approach has not worked, however, because BPA polycarbonate is not miscible with BPT-1 or BPT-2 polycarbonates.
It would be desirable to have a method for modifying the thermal characteristics of polycarbonate to increase the glass transition temperature, that does not result in a significant degradation of the impact properties of the polymer, and which can be carried out using blends of a limited number of feedstock polymers.
It has now been determined that a copolymers of BPA, BPT-1 and BPT-2, can be used as a modifier to enhance the thermal properties of a BPA polymer. A single copolymer modifier can be used in differing amounts to produce a product with desired thermal properties. Thus, the present invention provides a method for making a polycarbonate comprising the steps of (a) combining a bisphenol A polycarbonate and a BPA/BPT-1/BPT-2 copolymer modifier, and(b) mixing the combined materials to form a blend. By adjusting the relative amounts and the properties of the bisphenol A polycarbonate and BPA/BPT-1/BPT-2 copolymer, the glass transition temperature and the toughness of the blend can be selected. Suitable BPA/BPT-1/BPT-2 copolymers contain BPT-1 and BPT-2 in a ratio, BPT-1/BPT-2, in the range of 70/30 to 10/90, preferably less than or equal to 50/50, for example around 30/70.